1. Field of the Invention:
The present invention relates to freeze drying method and apparatus suitable for the treatment of material such as liquid solutions, emulsions, suspensions of solids in liquids, slurries and the like.
2. Description of the Prior Art:
In a conventional freeze drying method for the material to be dried, there is employed a tray/shelf system. In this system, the material having been frozen and received in containers such as trays and the like is disposed on or between shelves in a vacuum chamber, from which shelves a certain amount of heat is supplied to such frozen material so that sublimation of at least one of constituents of the frozen material occurs. After completion of such sublimation, clean air or nitrogen gas is introduced into the vacuum chamber. Then, the material having been dried through such sublimation is taken out of the vacuum chamber together with the containers.
In such conventional method, a mass-produced product, for example, a coffee extract liquid is first concentrated and then frozen. The thus frozen coffee extract is granulated to have a particle-size of from 1 to 3 mm. After that, the trays are filled with the thus granulated coffee extract. As for another mass-produced product, for example, a drug liquid, the bulk drying thereof is hitherto employed. In such bulk drying, the drug liquid is pressed out in a fine spray directed to a liquid of "Freon 12" which is one of trade names of dichlorodifluoromethane (CCl.sub.2 F.sub.2) so as to form a fine particle-size frozen matter with which the trays are filled. The above-mentioned conventional method will be hereinafter referred to as the prior art 1.
In another conventional freeze drying method for the liquid material to be dried, the material is first poured into the trays and then disposed on cooling shelves or disposed in a freezing chamber so that the material is frozen as is in cases of most bulk drying operations of the drugs and a few foods. Such another conventional freeze drying method will be hereinafter referred to as the prior art 2.
In any of the prior arts 1 and 2, the material to be dried is first spread on plate-like trays widely and thinly, and then subjected to a preliminary freezing operation and a freeze drying operation. After that, the trays are upset to collect the product. Consequently, in any case, it is necessary to handle a plurality of the trays each of which has a wide surface area, in a wide space by means of a complex handling mechanism or at the expense of considerable labors. In case that the material to be dried must be treated in a high-level hygienic environment, such treatment must be conducted in a bio-clean room.
Especially, in the prior art 2 in which the liquid material is first poured into the trays and then frozen in the trays, the material frozen in the trays can not be separated from the trays by simply upsetting the trays. Consequently, in this case, it is necessary to scrape the frozen material off the trays manually or by means of an automatic scraping mechanism. Such manual or automatic scraping operation of the material frozen in the trays makes the process of the prior art 2 complex. These are disadvantages inherent in the prior arts 1 and 2.
In order to eliminate these disadvantages inherent in the prior arts 1 and 2 or the tray/shelf system, there has been provided another conventional freeze drying method as shown in U.S. Pat. Nos. 3,281,956 (prior art 3) and 3,264,745 (prior art 4), in which: a space defined between upright cylinders is filled with the liquid material being dried, and then the surfaces of such cylinders are cooled to form a desired-thickness frozen layer of the liquid material on each of the surfaces of the cylinders. After completion of formation of such frozen layer of the liquid material, the remaining part of the liquid material is removed from the space defined between the cylinders. After that, the frozen layer of the material is subjected to a vacuum environment while heated through the surfaces of the cylinders to obtain from the surfaces the heat required for sublimation, so that sublimation of at least one of constituents of the materials occurs. After completion of such sublimation, the layers of the material dried on the surfaces of the cylinders through such sublimation are scraped from the surfaces of the cylinders and collected by a product receiver disposed below the cylinders. The conventional method disclosed in the above U.S. Patents will be hereinafter referred to as the prior art 3. More particularly, in the prior art 3, a heat-transfer medium is circulated through the cylinders so that the liquid material received in the space defined between these cylinders are frozen in positions adjacent to the surfaces of the cylinders to form the desired-thickness frozen layers of the material in such positions. Then, the remaining part of the liquid material still not frozen in the space is removed from the space, and thereafter the frozen layers of the material is subjected to the vacuum environment while gradually heated by means of the heat-transfer medium circulated in the cylinders. The thus obtained product having been dried on the surfaces of the cylinders are scraped from the surfaces of the cylinders by means of a scraper which rests at a position above the cylinders and is driven downward by a threaded rod in such scraping operation. The thus scraped product or dried material is collected by the product receiver.
As described above, in the prior art 3, the layers of material frozen on the surfaces of the cylinders adhere to the surfaces of the cylinders. Consequently, in order to separate the frozen layers of the material from the cylinders, in the prior art 3, there is employed a scraping mechanism comprising a disk-like scraper having a plurality of circular holes each of which has a diameter slightly larger than an outer diameter of each of the cylinders. In the scraping operation of the frozen material or product, the cylinders pass through the circular holes of the disk-like scraper in a sliding manner so as to scrape the product off the surfaces of the cylinders. Consequently, due to clearances between the circular holes of the scraper and the cylinders, thin layers of the product or frozen material remain on the surfaces of the cylinders, while a metal powder is produced due to a slidable contact established between the surfaces of the cylinders and the scraper, both of which are made of metal. These are disadvantages inherent in the prior art 3.
On the other hand, further another conventional method for freeze drying is disclosed in the prior art 4, in which: a heat-transfer medium for the cooling purpose is circulated through an outer space defined between a plurality of upright cylinders filled with the material being dried, so that a frozen layer of the material is formed on an inner surface of each of the cylinders in contrast with the prior art 3 in which the frozen layer of the material is formed on an outer surface of each of the cylinders. In the prior art 4, there is described that: any scraping mechanism is not employed, and, therefore, in order to facilitate separation of the product or dried material from the inner surfaces of the cylinders, any of the cylinders must be straight in shape and must be free from any deformation even when the temperatures of the cylinders vary.
In the prior art 4, it is described that the product dried on the inner surfaces of the cylinders can be easily separated from the inner surfaces of the cylinders, and therefore any scraping mechanism is not employed. In general, the frozen material is slightly contracted when dried, so as to facilitate separation of the thus dried material from the inner surfaces of the cylinders. However, in most cases, depending on the properties of the material being dried and conditions of the freezing and drying operations, the material having been received in the cylinders in a liquid state and then frozen therein tends to adhere to the inner surfaces of the cylinders except that the material is an extremely dilute solution. As a result, it is not possible to completely separate the dried material from the inner surfaces of the vertical cylinders, and, therefore, a part of the dried material rests on the inner surfaces of the cylinders. This is a defect inherent in the prior art 4.
In any of the prior arts 3 and 4, such remaining part of the dried material resting on the surfaces of the cylinders is subjected to the following cycle of the freeze drying operation of the liquid material, and thereafter repeatedly subjected to the further following cycles in the same manner. If the liquid material to be received in the cylinders during the next cycle of the operation is heated and the surfaces of the cylinders carrying the remaining part of the dried material are also heated, it is possible to dissolve the remaining part of the dried material adhering to the surface of the cylinders so as to remove the same from the surfaces of the cylinders. However, in any of the prior arts 3 and 4, heating of the liquid material deteriorates the quality of the product. In this connection, in the following cycle of the freeze drying operation, in case that the cylinders are kept at a temperature below 0.degree. C. at their surfaces while filled with the liquid material having a temperature of approximately 0.degree. C., the remaining part of the dried material formed on the surfaces of the cylinders during the previous cycle of the freeze drying operation remains as it is in the following cycle of the operation. The more the material is concentrated the more the material adheres to the surfaces of the cylinders. This is another defect inherent in the prior arts 3 and 4.